1. Field of the Invention
The present invention relates to a radio communication system, a radio communication method, a base station device, and a terminal device, and more particularly to a radio communication system, a radio communication method, a base station device, and a terminal device, for transmitting and receiving a synchronous signal in a system where a large number of antennas are dispersed in a cellular radio communication.
2. Description of Related Art
In a radio communication, for example, in a WiMAX system, there is a signal (synchronous signal) called “ranging” transmitted by a mobile device. A base station receives the ranging to be transmitted by the mobile device in an initial access, or to be transmitted periodically. The base station detects a delay of receiving timing of the ranging, and transmits a control signal for correcting the delay toward the mobile device. The mobile device has a mechanism for receiving a timing correction signal, and correcting the timing of a transmission signal.
Also, the mobile device transmits the ranging when transiting from a non-communication state to a communication state, and performs synchronization with a system timing.
As a background art of this technique, for example, there is IEEE802.16e (Non-patent Document 1). This standard document discloses the ranging which is transmitted by the mobile device. The standard document discloses two types of the transmission of the ranging in an asynchronous state and the transmission of the ranging in a synchronous state.
The ranging to be transmitted in the asynchronous state is transmitted mainly when transiting from the non-communication state to the communication state, and intended for synchronization with the system timing. The mobile device selects a specific code at random from plural codes each having a range designated by the base station in advance, and transmits the ranging of the code selected in the timing designated by the base station. In the timing when the base station receives the ranging instructed by the base station, the base station receives the signal transmitted by the mobile device. The base station does not know which code can be received, and therefore there is a need to conduct correlation operation with a received signal, and detect the received ranging for all of the codes. Also, the base station also detects a reception timing shift of the ranging. The base station measures whether the timing of the signal transmitted by the mobile device is delayed or advanced with respect to a timing expected by the base station. The base station then transmits a message for timing correction so that the reception timing falls within a correct range.
The ranging to be transmitted in the synchronous state is mainly transmitted in the communication state, and used for controlling the reception timing changed with the travel of the mobile device to be always kept accurate. The base station transmits an instruction for transmittable timing to the mobile device. Also, the base station designates a ranging code. The mobile device determines whether the ranging can be transmitted at the transmittable timing according to conditions such that another signal is not transmitted. If the ranging is transmittable, the mobile device transmits the ranging by using the designated ranging code.